As shown in FIG. 1, a conventional diffusion type plane diffusing screen 10 is configured such that light incident on the screen 10 is diffused in all directions. An image formed on the screen 10 is even, but has a reflectivity of 1% (generally, referred to as “1 gain”), thus being dark. Since the light incident on the screen 10 is diffused in all directions, the screen 10 acts on light from all directions. Accordingly, the screen 10 has a reduced resolution due to ambient light from all directions.
As shown in FIG. 2, a plane reflective screen 20 with an improved surface reflectivity is configured such that light projected from a projector is incident on the screen 20 at an incident angle (∠A) and then reflected from the screen 20 at a reflection angle (∠B) same as the incident angle (∠A), thus allowing a viewer to watch an image formed on the screen 2 in the range of C obtained by the reverse angle of the reflection angle (∠B).
That is, a hot spot is formed in the range of C. As shown in FIG. 2, the image displayed on the screen 2 is bright only at a central portion, but is dark at other peripheral portions, thus being invisible at the peripheral portions. In case that the surface reflectivity of the screen 2 is increased, the hot spot become brighter. On the other hand, in case that the surface reflectivity of the screen 2 is decreased, the hot spot will becomes darker and enlarger.
Further, in case that the surface reflectivity of the screen 2 is high, a screen having a non-spherical shape cannot display an image with even luminance, thus displaying uneven spots thereon. Further, since the screen having a non-spherical shape does not form an exact focus, this non-spherical screen cannot be used in a device requiring a high luminance.
However, all spherical screens cannot increase the luminance of screens. Only when a spherical screen comprises an optical element such as lens and a reflective surface, a screen can obtain an image with even luminance and high reflectivity over the whole surface of the screen.
Accordingly, there is required a method for forming an optical constitution and a surface reflectivity suitable for the structure of the screen.
Further, since the luminance of a screen is inversely proportional to the viewing angle of a screen, when a screen displays an image with a high luminance, the viewing angle of the screen is narrowed. The screen with the narrowed viewing angle cannot be effectively operated. Accordingly, there is required a spherical screen, which can display an image with a high luminance at a wider viewing angle.